Titanium dioxide pigments

ABSTRACT

This invention relates to the preparation of titanium dioxide pigment and has as its principal object the production of titanium dioxide pigment of improved tinting strength and tint tone. In particular, the present invention relates to a process for the preparation of TiO2 in pigmentary form which comprises forming a slurry of particulate TiO2 in hydrofluoric acid, allowing the slurry to stand until at least some of the slurried solids have been dissolved, and thereafter recovering the resulting titanium dioxide pigment of reduced particle size, improved tinting strength and bluer tint tone.

United States Patent [191 Lewis, Jr. et al.

[ TITANIUM DIOXIDE PIGMENTS [75] Inventors: Joseph Earle Lewis, Jr.,Brighton,

England; George Leathwhite Roberts, Jr., Lynchburg, Va.

[73] Assignee: American Cyanamid Company,

Stamford, Conn.

[22] Filed: July 30, 1970 [21] Appl. No.: 64,120

Related U.S. Application Data [63] Continuation of Ser. No. 849,280,Aug. 5', 1969, abandoned, which is a continuation of Ser. No. 662,595,Aug. 23, 1967, abandoned.

[52] U.S. CL 423/610, 106/300 [51] Int. Cl C0lg 23/04, COlg 23/08 [58]Field of Search.....'.. 23/202 R, 202 V; 106/300;

[56] References Cited UNITED STATES PATENTS 2,09l,955 9/1937 Allan23/202 R X [ll] 3,821,359 [451 June 28, 1974 2,304,l l0 l2/l942 McKinneyet a] 23/202 R 3,442,678 5/1969 Ross lO6/300 3,567,478 3/1971 Dietz eta] I06/300 Primary Examiner-Edward Stern Attorney, Agent, or Firm-RolandA. Dexter 5 7] ABSTRACT This invention relates to the preparation oftitanium dioxide pigment and has as its principal object the productionof titanium dioxide: pigment of improved tinting strength and tint tone.In. particular, the present 7 Claims, No Drawings 1 TITANIUM DIOXIDEPIGMENTS This Application is a streamlined continuation of US. PatentApplication, Ser. No. 849,280 filed Aug. 5, 1969, now abandoned, whichApplication in turn was a continuation of U.S. Pat. Application Ser. No.662,595, filed Aug. 23, 1967, also now abandoned.

This invention relates to the preparation of titanium dioxide pigmentand has as its principal object the production of titanium dioxidepigment of improved tinting strength and tint tone. More particularly,the invention is concerned with the wet finishing of titanium dioxidepigment utilizing chemical means. Still more par ticularly, theinvention relates to the wet finishing of titanium dioxide pigment inthe presence of hydrofluoric acid whereby the tinting strength and tinttone of the titanium dioxide pigment is improved without the need toresort to conventional mechanical milling procedures.

As is known, pigmentary titanium dioxide (TiO is commerciallymanufactured by one of two processes. One process involves thevaporphase oxidation of a titanium halide, such as titanium tetrachloride, toobtain a titanium dioxide product which may require a grinding ormilling treatment to obtain a smooth-textured pigment product of uniformparticle size. The other process for preparing pigmentary TiO involvesleaching a titaniferous ore with sulfuric acid, separating the titaniumsulfate and hydrolyzing it to obtain hydrous Ti which is then washed andfinally calcined. The product resulting from the calcination operationis not of uniform particle size and is conventionally subjected toeither a milling or a grinding operation to break down agglomerates andeffect particle size reduction and thereby improve the tinting strengthand tint tone of the Ti0 particles. However, these procedures are notwholly satisfactory as finishing operations. For instance, grindingdevices are costly to purchase and install. Large equipment can oftenhandle only relatively small capacities. Moreover, due to caking in themill, the particles are not all subjected to the same grindingconditions, and uniformity is not easily obtained. There is thus a needfor an improved means for producing TiO- of uniformly small particlesize and high tinting strength and desirable tint tone.

it is an object of this invention to provide a means for treating TiOfor pigmentary use without the need for mechanical grinding. It is afurther object of this invention to provide an improved method forobtaining pigmentary TiO of good tinting strength and tint tone.

In accordance with this invention there is provided a chemical, ratherthan mechanical technique for treating TiO This invention is based uponthe discovery that controlled used of aq ueous hydrofluoric acid caneffect desired particle size reduction and thus enhance the tint toneand tinting strength of the treated Ti0 material. The treatment withaqueous HF advantageously involves merely admixing the TiO and aqueousHF and allowing the mixture to age until the desired uniformity and sizereduction are achieved. Thereafter, the "H0 is filtered and washed.Further conventional finishing treatments may be applied if desired, andit is then dried and milled to give the product of improved properties.

The fundamental basis of the present invention is the provision of achemical means for reducing aggregate size. This is somewhat akin to thepractice of breaking aggregates and floccules of organic pigments by theuse of dispersing agents to reduce the crystal energy. In this case, amuch stronger chemical attack is necessary since the high temperatureheat treatment of the TiO formation process gives aggregates which arehighly sintered and very difficult to break up by the use ofconventional milling equipment and dispersing agents. The critical partof the invention is that the combination of acid concentration and thetime and temperature of treatment must be such that a portion of thesolid is dissolved.

Higher acid concentrations, longer periods of treatment and highertemperatures generally lead to the solution of greater amounts ofmaterial and increased particle size reduction. Conditions should beadjusted so that the desired degree of size reduction will be obtainedin the planned time of acid treatment. Even dilute HF can be used if theTiO -l-IF slurry is heated for a long time. However, it is preferred touse HF of at least 15 percent concentration so that the desiredreduction can be effected in a reasonable time, i.e., in less than 48hours. It is likewise desirable for practical purposes to adjust acidstrength to a level such that particle size reduction can be effectedwithin the range of room temperature to the boiling temperature atatmospheric pressure of the TiO slurry. Accordingly, for practicalpurposes, when using an acid solution of 25-50 percent concentration, itis preferred to employ from about 1 to 10 parts of acid for each part ofTi0 starting material, and to conduct the acid treatment at atemperature of 20C. to 95C. for about 15 minutes to 24 hours. Since, asnoted above, the desired particle size reduction obtained by thepractice of this invention is the result of the dissolving action of thehydrofluoric acid, it is possible to express the criteria of treatmentin terms of the amount of TiO which is dissolved. In general, forreasons of economy, treatment should not result in the dissolution ofmore than 50 percent of the TiO starting material. On the other hand, ifless than about 1 percent of the TiO starting material is dissolved,then the amount of size reduction may be insufficient. Dissolution inthe range of 5-50 percent, and preferably less than 40 percent, of theTiO starting material, will normally mean that the desired degree ofparticle size reduction has been achieved with only a minor sacrifice ofeconomy. However, the TiO which is dissolved, can be recovered byprecipitation technique and reprocessed to finished pigment by severalroutes.

Advantageously, the presence of other materials, such as nitric acid,oxalic acid, acetic acid or acetyl acetone, usually from 1 percent topercent (depending upon the material) based on the weight ofhydrofluoric acid, can, if desired, be added to the hydrofluoric acidtreating solution so as to dilute the latter. Notwithstanding suchdilution, good tint tones of the resultant titanium dioxide pigment areobserved.

The tinting strength of the pigment is determined by admixing thepigment with castor oil, linseed oil or ricinoleic acid, or anequivalent thereof, to form a paste and then adding carbon black inincrements until the paste sample matches a standard paste prepared fromcommercial titanium dioxide and a specified amount of carbon black. Thetinting strength is reported as a number which represents the relativeoptical efficiency of the pigments compared to a sample of basiccarbonate of white lead which was rated at 100. The tint tone representsthe observed difference from the standard in terms of brownness (Br) orblueness (B1) on a scale from Br6 to B1 6. A tint tone shift from brownto blue indicates a decrease in the diameter of the pigment particles.

As hereinabove stated, the hydrofluoric acid treatment of the presentinvention is equally applicable to titanium dioxide pigments obtained bythe calcination of hydrolysates of titanium sulfate solutions, and topigments obtained by the oxidation of titanium tetrachloride. Theproducts obtained from both processes contain substantial amounts ofcoarse particles and both can be improved by the practice of thisinvention. The process can also be used to augment conventionalprocesses aimed at effecting particle size reduction. Thus instead ofreplacing a milling operation with acid treatment, it is possible to addacid either during milling, or after milling, to the milled product.

The following Examples are presented to illustrate the preferredpractice of this invention. They are merely illustrative and are not tobe taken as limiting the invention. Unless otherwise stated, parts areby weight.

EXAMPLE 1 Twenty grams of micropulverized calciner discharge wereslurried with 100 ml. of 49.5 percent aqueous hydrofluoric acid. Themixture was allowed to stand at room temperature for three hours. Theaged slurry was filtered, and the cake was washed with water andovendried. The tinting strength and tint tone of the dried pigment weredetermined to be 1,500 and Br3, respectively. The calcined titaniumdioxide before treatment exhibited a tinting strength and tint tone as1,420 and Br6, respectively.

EXAMPLE 2 The procedure of Example 1 was repeated in every detail exceptthat the slurried mixture was allowed to stand for five hours. Thetinting strength and tint tone of the finished product were: 1,550 andBrl, respectively.

EXAMPLE 3 30 Grams calcined titanium dioxide having a tinting strengthand tint tone of 1,420 and Br6, respectively, were slurried with 105 ml.of aqueous 49.5 percent hydrofluoric acid and 5 ml. of water and allowedto stand for 18.5 hours at room temperature. The aged slurry wasfiltered and the cake was washed with water and oven-dried. Examinationof the treated titanium dioxide revealed a tinting strength and a tinttone of 1645 and 81-2, respectively.

EXAMPLE 4 Repeating the procedure of Example 3 in every detail, exceptfor theuse of ml. of the aqueous hydrofluoric acid and a treatment timeof 21 hours, there was obtained a T10 product having a tinting strengthand tint tone of 1650 and Bl1 respectively.

EXAMPLE 5 tinting strength and tint tone of 1620 and 81-2, respectively,as compared to 1420 and Br6 for the untreated calcined TiO pigment.

EXAMPLE 6 Separate portions (20 grams each) of TiO slurry prepared bythe procedure of Example 5 were treated with (a) 5 g. ztcetyl acetone,(h) 5 g. oxnlic ncid and (c) 10 g. dextrose. respectively. The productwas filtered. and the cake was washed as in Example 5. The resultantpigments had the following tinting strengths and lint tones: (a) 1630and 131-2, (b) 1610 and Bl-2 and (c) 1610 and Bll.

EXAMPLE 7 5 g. TiO were slurried with a mixture of 1.25 ml. of 49percent HF and 7 ml. of concentrated nitric acid for 16.5 hours at C.The resultant mixture was filtered, and the cake was water-washed anddried. The pigment thus obtained possessed a tinting strength and tinttone of 1500 and Bl-2, respectively, as compared to a tinting strengthand a tint tone of 1420 and Br6, respectively, for the untreatedpigment.

EXAMPLE 9 Ten g. micropulverized calciner discharge were added to m1. of49.5 percent HF and allowed to stand for 22 hours at room temperature.The aged slurry was filtered, the cake was washed with water and driedat C. 0.9 g. of solid was obtained (approximately 9 percent).

EXAMPLE 10 A sample of anatase calciner discharge (60 g.) was slurriedin 30 ml. water in a polyethylene beaker at 8090C. Hydrofluoric acid (50ml. of 49.5 percent acid) was then, added and the resulting flocculatedslurry was allowed to remain in the bath for one hour. The aged slurrywas filtered and washed with water at room temperature. The tintingstrength and tint tone reported of the residue (40 g.) were 1300 andBl-4, respectively, compared to 1320 and Br-2 for the starting material.

EXAMPLE 1 l A sample of TiO prepared by the oxidation of titaniumtetrachloride (40 g.) at high temperature, was slurried in 50 ml.hydrofluoric acid (49.5%) and allowed to stand at room temperature for19 hours. The residue of titanium dioxide (24 g.) was washed with water.The tinting strength and tint tone of this residue were 1650 and Bl-3compared to 1630 and Br-2 for the starting material.

EXAMPLE 12 A sample of TiO prepared by the calcination of a hydroustitanium dioxide (50 g.) was slurried in 50 ml. of

49.5% hydrofluoric acid and immediately heated at We claim:

l. A process for the wet finishing of anhydrous TiO in pigmentary formwhich comprises slurrying calcined anhydrous titanium dioxide pigmentaryparticles in an aqueous solution of to 50 percent by weight ofhydrofluoric acid, allowing said slurry to age for at least 15 minutesat a temperature of from to 80C until from 5 percent to 50 percent byweight of the slurried titanium dioxide hasbeen dissolved, thereafterdirectly filtering the residual titanium dioxide pigment from saidstrongly acid slurry and washing said pigment whereby a pigment ofreduced particle size and improved tinting strength and bluer tint toneis obtained.

2. The process according to claim 1 wherein the hydrofluoric acid has aconcentration of percent to percent.

3. The process according to claim 1 wherein from one to ten parts ofacid are used for each part of TiO pigment.

4. The process according to claim 1 wherein conditions are such that 5 5percent of the Ti0 is dissolved by the hydrofluoric acid solution.

5. The process according to claim I wherein a physical milling treatmentis employed in conjunction with said hydrofluoric acid treatment.

6. The process according to claim 5 wherein the treatment withhydrofluoric acid is conducted simulta neously with said millingtreatment.

7. The process according to claim 5 wherein the treatment withhydrofluoric acid is prior to said milling treatment.

2. The process according to claim 1 wherein the hydrofluoric acid has aconcentration of 35 percent to 50 percent.
 3. The process according toclaim 1 wherein from one to ten parts of acid are used for each part ofTiO2 pigment.
 4. The process according to claim 1 wherein conditions aresuch that 5-15 percent of the TiO2 is dissolved by the hydrofluoric acidsolution.
 5. The process according to claim 1 wherein a physical millingtreatment is employed in conjunction with said hydrofluoric acidtreatment.
 6. The process according to claim 5 wherein the treatmentwith hydrofluoric acid is conducted simultaneously with said millingtreatment.
 7. The process according to claim 5 wherein the treatmentwith hydrofluoric acid is prior to said milling treatment.